I'm approaching 3k miles on my 06 Limited V8 4WD, and I've recorded every gas purchase since I took delivery.
The lowest mileage on a tankful was 14.5 mpg, when I drove mostly city traffic, some off-road, and some pulling a utility trailer with a lawn tractor.
The highest mileage thus far for a tankful has been 20.0 mpg, which was primarily interstate driving using cruise control and staying around 70mph.
For the entire life of the vehicle, my average mileage is now 17.2 mpg. I haven't pampered the truck, but I don't have to be first off the line when the light turns green either. I've been using various brands of 87 octane, and just one tankful of 93 octane (and I saw no difference in mileage or performance.)